Clad structural member with NbTiAl low Hf alloy cladding and niobium base metal core

ABSTRACT

Composite structures having a higher density, stronger reinforcing niobium based alloy embedded within a lower density, lower strength niobium based cladding alloy are provided. The cladding is preferably an alloy having a niobium and titanium base according to the expression: 
     
         Nb.sub.balance --Ti.sub.40-48 --Al.sub.12-22 --Hf.sub.0.5-6. 
    
     The reinforcement may be in the form of plates, sheets or rods of the higher strength, higher temperature niobium based reinforcing alloy. The same crystal form is present in both the matrix and the reinforcement and is specifically body centered cubic crystal form.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to copending U.S. patent applications:

Ser. No. 907,949, filed Jul. 2, 1992; Ser. No. 816,164, filed Jan. 2,1992; and Ser. Nos. 07/814,794, 07/815,797, 07/816,161, and 07/816,165,all filed Jan. 2, 1992;

Ser. No. 953,702, filed Sep. 30, 1992; Ser. No. 953,701, filed Sep. 30,1992; Ser. No. 953,911, filed Sep. 30, 1992; Ser. No. 953,907, filedSep. 30, 1992; and Ser. No. 953,910, filed Sep. 30, 1992.

BACKGROUND OF THE INVENTION

The present invention relates to composite metal structures in which ametal cladding having a lower density and a lower tensile strength athigh temperature is reinforced by a core of a metal present in highervolume fraction and having both higher tensile strength and higherdensity than that of the cladding. The invention further relates to thereinforcement of lower density metal clad structures having a niobiumtitanium base cladding and a higher oxidation resistance, with metalcore elements having a lower oxidation resistance as well as higherdensity and higher strength.

The invention additionally relates to body centered cubic metalstructures in which a metal cladding having a lower density and a lowertensile strength at high temperature is reinforced by core elements of ametal present in higher volume fraction and having both higher tensilestrength and higher density than that of the cladding. Lastly, theinvention relates to metal-metal composite structures in which a lowerdensity metal clad having a niobium titanium base and a higher oxidationresistance is reinforced with denser, but stronger, niobium base metalreinforcing elements having a lower oxidation resistance.

It is known that niobium base alloys have useful strength in temperatureranges at which nickel and cobalt base superalloys begin to showincipient melting. This incipient melting temperature is in theapproximately 2300° to 2400° F. range. The use of the higher meltingniobium base metals in advanced jet engine turbine hot sections wouldallow higher metal temperatures than are currently allowed. Such use ofthe niobium base alloy materials could permit higher flame temperaturesand would also permit production of greater power at greater efficiency.Such greater power production at greater efficiency would be at least inpart due to a reduction in cooling air requirements.

The commercially available niobium base alloys have high strength andhigh density but have very limited oxidation resistance in the range of1600° F. to 2400° F. Silicide coatings exist which might offer someprotection of such alloys at temperatures up to 2400° F., but suchsilicide coatings are brittle enough that premature failure of thecoating could be encountered where the coated part is highly stressed.The commercially available niobium base alloys also have high densitiesranging from a low value of 8.6 grams per cubic centimeter forrelatively pure niobium to values of about 10 grams per cubic centimeterfor the strongest alloys.

Certain alloys having a niobium-titanium base have much lower densitiesof the range of 6-7 grams per cubic centimeter. A group of such alloysare the subject matter of commonly owned U.S. Pat. Nos. 4,956,144;4,990,308; 5,006,307; 5,019,334; and 5,026,522. Such alloys can beformed into parts which have significantly lower weight than the weightof the presently employed nickel and cobalt superalloys as thesesuperalloys have densities ranging from about 8 to about 9.3 grams percubic centimeter. One of these patents, U.S. Pat. No. 5,006,307,concerns an alloy having the following composition in atom percent:

    ______________________________________                                                      Concentration                                                   Ingredient    Range                                                           ______________________________________                                        niobium       balance                                                         titanium      40-48%                                                          aluminum      12-22%                                                          hafnium       0.5-6%                                                          ______________________________________                                    

A number of additional niobium based alloys are also the subject ofcommonly owned U.S. Patents. These patents are U.S. Pat. Nos. 4,890,244;4,931,254; 4,983,356; and 5,000,913. This latter group of alloys hasuniquely valuable sets of properties but has densities which are higherthan those of the other alloys. Commonly owned U.S. Pat. No. 4,904,546concerns an alloy system in which a niobium base alloy is protected fromenvironmental attack by a surface coating of an alloy highly resistantto oxidation and other atmospheric attack. Commonly owned copendingapplications, listed in the cross-reference section above pertainprincipally to matrix type composites in which reinforcement and matrixalloys are intimately intermixed and the relevant reinforcement ratios,as defined below, is above 50 and preferably above 100. By contrast thecomposites of the invention have reinforcement ratios below 50.

In devising alloy systems for use in aircraft engines the density of thealloys is, of course, a significant factor which often determineswhether the alloy is the best available for use in the engineapplication. The nickel and cobalt based superalloys also have muchgreater tolerance to oxygen exposure than the commercially availableniobium based alloys. The failure of a protective coating on a nickel orcobalt superalloy is a much less catastrophic event than the failure ofa protective coating on many of the niobium based alloys andparticularly the commercially available niobium based alloys. Theoxidation resistance of the niobium based alloys of the above commonlyowned patents is intermediate between the resistance of commercial Nbbase alloys and that of the Ni- or Co-based superalloys.

While the niobium based alloys of the above commonly owned patents arestronger than wrought nickel or cobalt based superalloys at hightemperatures, they are much weaker than cast or directionally solidifiednickel or cobalt based superalloys at these higher temperatures.However, for many engine applications, structures formed by wroughtsheet fabrication are used, since castings of sheet structures cannot beproduced economically in sound form for these applications.

The advantage of use of niobium based structures is evidenced by thefact that the niobium based alloys can withstand 3 ksi for 1000 hours attemperatures of 2100° F. The nickel and cobalt based wroughtsuperalloys, by contrast, can withstand 3 ksi of stress for 1000 hoursat only 1700° to 1850° F.

What is highly desirable in general for aircraft engine use is astructure which has a combination of lower density, higher strength athigher temperatures, good ductility at room temperature, and higheroxidation resistance. We have devised metal-metal clad compositestructures which have such a combination of properties.

A number of articles have been written about use of refractory metals inhigh temperature applications. These articles include the following:

(1) Studies of composite structures of tungsten in niobium wereperformed at Lewis Research Center by D. W. Petrasek and R. H. Titranand are reported in a report entitled "Creep Behavior ofTungsten/Niobium and Tungsten/Niobium-1 Percent Zirconlure Composites"and identified as Report No. DOE/NASA/16310-5 NASA TM-100804, preparedfor Fifth Symposium on Space Nuclear Power Systems, University of NewMexico, Albuquerque, N. Mex. (Jan. 11-14 1988). No studies ofreinforcing niobium base matrices with niobium base structures, nor theunique benefits of such reinforcing, is taught in this report.

(2) S. T. Wlodek, "The Properties of Cb--Ti--W Alloys, Part I",Oxidation, Columbium Metallurgy, D. Douglass and F. W. Kunz, eds., AIMEMetallurgical Society Conferences, vol. 10, Interscience Publishers, NewYork (1961) pp. 175-204.

(3) S. T. Wlodek, "The Properties of Cb--Al--V Alloys, Part I",Oxidation, ibid., pp. 553-584.

(4) S. Priceman and L. Sama, "Fused Slurry Silicide Coatings for theElevated Temperature Oxidation of Columbium Alloys", Refractory Metalsand Alloys IV-TMS Conference Proceedings, French Lick, IN, Oct. 3-5,1965, vol. II, R. I. Jaffee, G. M. Ault, J. Maltz, and M. Semchyshen,eds., Gordon and Breach Science Publisher, New York (1966) pp. 959-982.

(5) M. R. Jackson and K. D. Jones, "Mechanical Behavior of Nb--Ti BaseAlloys", Refractory Metals: Extraction, Processing and Applications, K.C. Liddell, D. R. Sadoway, and R. G. Bautista, eds., TMS, Warrendale,Pa. (1990) pp. 311-320.

(6) M. R. Jackson, K. D. Jones, S. C. Huang, and L. A. Peluso, "Responseof Nb--Ti Alloys to High Temperature Air Exposure", ibid., pp. 335-346.

(7) M. G. Hebsur and R. H. Titran, "Tensile and Creep Rupture Behaviorof P/M Processed Nb--Base Alloy, WC-3009", Refractory Metals:State-of-the-Art 1988, P. Kumar and R. L. Ammon, eds., TMS, Warrendale,Pa. (1989) pp. 39-48.

(8) M. R. Jackson, P. A. Siemers, S. F. Rutkowski, and G. Frind,"Refractory Metal Structures Produced by Low Pressure PlasmaDeposition", ibid., pp. 107-118.

BRIEF STATEMENT OF THE INVENTION

In one of its broader aspects, objects of the present invention can beachieved by embedding at least one reinforcing structures of a niobiumbase metal of higher density, greater high temperature tensile strengthand lower oxidation resistance within a niobium base clad metal of lowerdensity, lower strength and higher oxidation resistance having thefollowing composition in atom percent:

    Nb.sub.balance --Ti.sub.40-48 --Al.sub.12-22 --Hf.sub.0.5-6,

where each metal of the metal/metal clad composite has a body centeredcubic crystal structure.

In another of its broader aspects, objects of the present invention canbe achieved by embedding at least one niobium base metal reinforcingmember, having a body centered cubic crystal form and having higherdensity and greater high temperature strength as well as a loweroxidation resistance, in a sheath having a niobium titanium base andhaving lower density, lower strength and higher oxidation resistance andhaving the following composition:

    Nb.sub.balance --Ti.sub.40-48 --Al.sub.12-22 --Hf.sub.1.5-5.

The reinforcement ratio of the structure is no more than 50 as explainedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

The description which follows will be understood with greater clarity ifreference is made to the accompanying drawings in which:

FIG. 1 is a photomicrograph of the cross section of a billet prepared bycoextruding elements;

FIG. 2 is a graph in which grain size of a matrix and of the embeddedreinforcement is plotted against heat treatment temperature;

FIG. 3 is a graph in which composite room temperature elongation isplotted against heat treatment temperature;

FIG. 4 is a graph in which composite room temperature elongation isplotted against grain size;

FIG. 5 is a graph in which composite yield strength is plotted againsttesting temperature;

FIG. 6 is a graph in which composite elongation to failure is plottedagainst testing temperature;

FIG. 7 is a Larson-Miller graph in which comparative data is givenregarding the stress rupture life of the composites;

FIG. 8 is a micrograph of a cross section of a continuous compositestructure; and

FIG. 9 is a graph in which yield strength is plotted against testtemperature.

DETAILED DESCRIPTION OF THE INVENTION

Pursuant to the present invention, clad composite structures are formedincorporating at least one strong, ductile metallic reinforcing elementin a ductile, low density, more oxygen-resistant sheath to achievegreater high temperature tensile and rupture strengths than can beachieved in an article of the same dimensions formed solely of thesheath metal by itself and to avoid oxidative degradation of thereinforcing element or elements.

Both the reinforcement composition and the sheath composition are highin niobium metal. Further, both the sheath and the reinforcement havethe same general crystalline form and specifically a body centered cubiccrystal structure. In this way, many of the problems conventionallyrelated to incompatibility of or interaction between a conventionalreinforcement and a conventional cladding to form brittle intermetallicsor other undesirable reaction products are deemed to be avoided. If acomposite of this invention containing a sheet form of reinforcement isheated for long times at high temperature, the sheet and its claddingare mutually soluble so that even a high degree of interdiffusion doesnot result in embrittlement. However, where clad composites of thisinvention are used for normal service lives and temperatures, verylittle interdiffusion and very little degradative alteration of therespective properties of the cladding and reinforcement are deemedlikely.

As used herein, the term "cladding" is meant to designate a relativelythin continuous layer of the metal having the lower density and higherresistance to oxidation. The cladding must be of sufficient thickness sothat oxygen cannot readily penetrate the cladding layer to interactdeleteriously with the surface of the reinforcement beneath thecladding. Also, in general, the reinforcement layer must have asufficient bulk and thickness to provide a reinforcing function so thatthe strength of the clad composite article is greater than a structurehaving the same volume of metal but formed only of the more oxidationresistant material. Further, the cladding portion of the clad compositeis sometimes referred to by other terms such as "sheath" or "envelope",or the like, but the meaning is essentially the same as that of"cladding". In some respects, the terminology employed has something todo with the manner in which the clad composite is formed. For example,if the clad composite is formed by coextrusion of elements as, forexample, in forming a rod-like element, a term, such as "envelope","cladding", or "sheath" may be appropriate to describe the protectivelayer of niobium base metal which envelops the reinforcing core. Asimilar designation would apply also to a clad composite strip formed bya coextrusion.

The thickness required for a cladding in order to prevent excessiveinterdiffusion and loss of the benefit of the cladding depends on theprojected temperature of use of the clad composite article. Therelationship between thickness and diffusion is explained more fullybelow.

In general, the fabrication techniques for forming such clad compositesinvolve embedding at least one higher strength, higher density ductileniobium base alloy body in an enveloping sheath of the lower density,lower strength ductile niobium base alloy and forming and shaping thecombination of materials into a clad composite body. In this way, it ispossible to form a clad composite which is strengthened by the greaterhigh temperature strength of the higher density core niobium alloy andwhich clad composite also enjoys the environmental resistance propertiesof the weaker cladding material.

The following examples illustrate some of the techniques by which thecomposites may be prepared and the properties achieved as a result ofsuch preparation. While these examples relate principally to matrix typecomposites of the cross-referenced applications, the advantages achievedare deemed to apply as well to clad type composites of this invention.

EXAMPLES 1 and 2

Two melts of matrix alloys were prepared and ingots were prepared fromthe melts. The ingots had compositions as listed in Table I immediatelybelow.

                  TABLE I                                                         ______________________________________                                        Matrix Alloy 108:                                                                         40 Nb   40 Ti    10 Al 8 Cr   2 Hf                                Matrix Alloy 124:                                                                         49 Nb   34 Ti     8 Al 7 Cr   2 Hf                                ______________________________________                                    

The alloys prepared were identified as alloys 108 and 124. Thecomposition of the alloys is given in Table I in atom percent. The alloy108 containing 40 atom percent titanium and 40 atom percent niobium is amore oxygen resistant or oxygen tolerant alloy, and the matrix alloyidentified as alloy 124 containing 34 atom percent titanium and 49 atompercent niobium is the stronger of the two matrix alloy materials athigh temperature.

A Wah Chang commercial niobium based reinforcing alloy was obtainedcontaining 30 weight percent of hafnium and 9 weight percent of tungstenin a niobium base. The alloy was identified as WC3009.

A cast ingot of each of the matrix alloy compositions was first preparedin cylindrical form. Seven holes were drilled in each of the ingots ofcast matrix alloy to receive seven cylinders of the reinforcingmaterial. The seven holes were in an array of six holes surrounding acentral seventh hole. Each of the reinforcing cylinders to be insertedin the prepared holes was formed of the WC3009 metal and was 0.09 inchin diameter and 2.4 inches in length. Seven dimensionally conformingcylinders were placed in the 7 drilled holes in each of the cast matrixalloy samples. Each assembly was then enclosed in a jacket of molybdenummetal and was subjected to an 8 to 1 extrusion reduction.

After the first extrusion, a three inch length was cut from the extrudedcomposite billet and the three inch length was placed in a secondconforming molybdenum jacket and subjected to a second extrusionoperation to produce an 8 to 1 reduction. Total cross-sectional areareduction of the original billet was 64 to 1.

A photomicrograph of the cross section of a twice extruded billet and ofthe contained reinforcing strands is provided in FIG. 1.

Seven sections were cut from the twice extruded billet and each sectionwas accorded a four hour heat treatment in argon at temperatures asfollows: 815° C.; 1050° C.; 1100° C.; 1150° C.; 1200° C.; 1300° C.; and1400° C.

Grain size measurements were made for both the reinforcing fiber and thematrix on each of these sections of the extruded billet. The initialgrain sizes of the matrix portions of the billet sections prior to heattreatment were less than 20 μm. The initial grain sizes were grown to 50to 100 μm by the 1100° C. heat treatment and to 200 to 300 μm by the1400° C. heat treatment. The matrix having the higher titaniumconcentration displayed the greater grain growth.

The grain size in the reinforcing WC3009 fiber could not be measuredoptically for the asextruded fiber nor could it be measured for thefiber after the 815° C. heat treatment. The grain size was about 5 μmfor the WC3009 fiber which had been treated at the 1050° C. temperature.The grain size of the fiber was less than 25 μm for the sample which hadbeen heat treated at 1400° C.

A plot of data concerned with grain size in relation to heat treatmenttemperature is set forth in FIG. 2.

The interface between the fiber and the matrix and the grain boundariesin the fiber were heavily decorated with precipitates of hafnium oxide(HfO₂). It is presumed that the oxygen in the matrix casting and on thefiber surfaces as well as on the matrix machined surfaces reacted withthe high hafnium concentrations in the WC3009 fibers.

Mechanical test bars were machined from the twice extruded compositesafter heat treatment at the 1100° C., 1200° C., and 1300° C. heattreatment temperatures. The test bar gage was 0.08 inches in diameterwith the outer gage surface of the matrix being approximately 0.005inches beyond the outer fiber surface, i.e., each fiber was at least0.005 inches from the outer surface of the matrix member. The sevenfibers were in a close-packed array having six outer fibers surroundinga central fiber on the axis of the test bar as illustrated in FIG. 1.All of the fibers were included within the 0.08 inch gauge diameter ofthe test bar. Tests were made of the bars as indicated in Table IIimmediately below:

                                      TABLE II                                    __________________________________________________________________________    Tensile Test Data for Composite of Continuous Fibers                          of WC3009 in Alloy Matrix                                                        Matrix                                                                              Heat Treatment                                                                        Test Temp                                                                           0.2% YS                                                                            UTS                                                                              EL.sub.ml                                                                        EL.sub.f                                                                         RA                                       Ex.                                                                              Alloy Temperature                                                                           (°C.)                                                                        (ksi)                                                                              (ksi)                                                                            (%)                                                                              (%)                                                                              (%)                                      __________________________________________________________________________    1  Matrix 108                                                                          1200 C. RT    128  128                                                                              0.2                                                                              23 36                                                        760   81   83 0.7                                                                              24 50                                                        980   22   24 0.6                                                                              40 70                                                        1200  10   11 0.8                                                                              39 96                                       2  Matrix 124                                                                          1200° C.                                                                       RT    131  131                                                                              0.2                                                                              22 35                                                        760   83   92 1.8                                                                              13 14                                                        980   35   35 0.2                                                                              59 76                                                        1200   9   14 1.4                                                                              53 95                                       1  Matrix 108                                                                          1100° C.                                                                       RT    126  127                                                                              0.3                                                                              26 37                                                1300° C.                                                                       RT    No Yield                                                                           40 0.02                                                                             0.02                                                                             0                                        2  Matrix 124                                                                          1100° C.                                                                       RT    134  134                                                                              0.2                                                                              26 45                                                1300° C.                                                                       RT    126  127                                                                              0.2                                                                              3.4                                                                              6.6                                      __________________________________________________________________________      In the above table:                                                          YS designates yield strength in ksi (1000 pounds/in.sup.2).                   UTS designates ultimate tensile strength in ksi.                              EL.sub.ML designates elongation (or strain) at maximum load (also known a     uniform strain) in percent.                                                   EL.sub.F designates elongation (or strain) at failure (also known as          fracture strain).                                                             RA designates reduction of area in percent.                              

It will be observed from the results listed in Table II that theductility of samples heat treated at 1300° C. decreased sharply whencompared to the ductility values achieved following heat treatment at1100° C. or 1200° C.

A plot of the data relating room-temperature elongation at failure toheat treatment temperature as set forth in Table II is presented in FIG.3. A plot relating grain size to room-temperature elongation at failureis presented in FIG. 4.

Tensile strengths were essentially in conformity with a rule of mixturescalculation for the respective volume fractions of fiber and matrix. Thevolume fraction of the materials tested to produce the results listed inTable II were about 15.8 volume percent of the WC3009 reinforcingfibers, each of which had a diameter measurement of about 0.012 inchesin the test bars subjected to testing. For the samples heat treated at1100° C. and at 1200° C., both composites exhibited room temperatureductilities of about 22% elongation with about a 35% reduction in area.It was observed that these ductilities were surprisingly high whencompared to values of 7-12% typical of similar matrix compositions whichcontained no fibers. It is known that the WC3009 alloy is generally lowin ductility, in the range of about 5% in a bulk form at roomtemperature, although the data which is available is only for the alloywith much coarser grain structures.

Data relating yield strength to temperature is plotted in FIG. 5 anddata relating percent elongation at failure to temperature for eachcomposite is plotted in FIG. 6.

Rupture data for the continuous composite of WC3009 continuous fibers inthe niobium based matrices were obtained by measurements made in anargon atmosphere at 982° C. essentially as listed in Table IIIimmediately below:

                  TABLE III                                                       ______________________________________                                        Rupture Life Data at 982° C. for                                       15.8 v/o WC3009 Filament in Reinforced Composites                                  Continuous                                                                    Composite Heat                       Rupture                                  with      Treatment  Stress                                                                              EL.sub.f                                                                           RA   life                                Ex.  Matrix    Temperature                                                                              (ksi) (%)  (%)  (hours)                             ______________________________________                                        1    108       1100° C.                                                                          9     64   82   23.3                                     108       1200° C.                                                                          12    No   No   0.6                                                                 Data Data                                     2    124       1100° C.                                                                          9     81   89   20.8                                     124       1200° C.                                                                          9     63   63   114.3                                    124       1300° C.                                                                          9     56   79   43.1                                ______________________________________                                    

As a matter of comparison, unreinforced alloys similar to the 108 matrixexhibit a rupture life at 985° C. of less than 25 hours at a stress ofonly 6 ksi. Correspondingly, an unreinforced alloy similar to the 124matrix exhibited a life of 1.8 hours at 9 ksi.

For reinforced structures as described above, the best composite testlife at equal stress was nearly 10 fold greater than the rupture life ofa similar unreinforced composition.

The densities for the two composites are approximately 7 grams per cubiccentimeter for the composite with the 108 matrix and 7.2 grams per cubiccentimeter for the composite with the 124 matrix. Comparable densityvalues for nickel and cobalt based alloys are 8.2 to 9.3 grams per cubiccentimeter. Although the composites are much stronger in rupture thanare wrought Ni and Co-base superalloys, the composites are still weakerthan cast γ/γ' superalloys. The density reduced stress for 100 hours at985° C. for the 124 composite is 1.25 (arbitrary units, ksi/g/cc), lessthan for cast alloys such as Rene 80 (density reduced stress of 1.84),but is much closer than is the case for unreinforced matrices(density-reduced stress of 0.75).

Rupture data obtained by measurements made in argon atmosphere at othertemperatures are listed in Table IV immediately below:

                  TABLE IV                                                        ______________________________________                                        Rupture Life Data for                                                         15.8 v/o WC3009 Filament in Reinforced Composites                             Continuous                                                                    Composite  Heat       Rupture Life (hours At)                                      with      Treatment  871° C. &                                                                     1093° C.                                                                      1149° C.                       Ex.  Matrix    Temperature                                                                              15 ksi & 5 ksi                                                                              & 3 ksi                               ______________________________________                                        1    108       1100° C.                                                                          34.3   11.5   60.3                                  2    124       1100° C.                                                                          81.6   16.1   500.5                                      124       1300° C.                                                                          46.2   42.2   372.1                                 ______________________________________                                    

Typical wrought Ni and Co superalloys would last less than 100 hours at1000° C. and 3 ksi. In terms of temperature capability, the reinforcedcomposites having the niobium-titanium base matrices would survive foran equivalent time and stress at a temperature 80° C. to 200° C. hotterthan wrought Ni or Co alloys.

Data concerning the stress rupture life of the composites as describedabove are set forth in the Larson-Miller plot of FIG. 7.

It is evident from the property improvements achieved in the aboveexamples that very good bonding is achieved between the matrix andreinforcing metals. Both of the ingredient metals of the matrixcomposites are ductile and both are of the body-centered cubic crystalform. Because of this high degree of compatibility between the matrixand the reinforcing components of the matrix composites, excellentcomposites are formed and very significant property improvements areachieved.

The applicants deem these compatibility factors and property improvementfactors to be evidence of compatibility and property improvement in cladcomposites. The distinction between matrix composites of copendingapplications referenced above and the clad composites of the subjectapplication is the degree of subdivision of the reinforcing component ofthe composite. In the case of the matrix composites, the degree ofsubdivision is high and consequently there is a large surface area ofthe reinforcing component of the matrix composites.

By contrast, in the clad composites of the subject invention, thereinforcing component has a simple geometric form such as a sheet orstrip and consequently the surface area of the reinforcing component ofthe clad composites is relatively small when compared to the surfacearea of the reinforcing component of the matrix composites. Thedistinction between these two factors is discussed more extensivelybelow.

The reinforcing metal of the above examples is not limited to thespecific alloys employed in those examples. Some niobium base alloys,other than WC3009, which are suitable for use as core strengtheningmaterials for clad composites include, among others, the following:

    ______________________________________                                        Other Commercially Available Niobium Base Alloys                              Useful as Strengthening Elements for the Niobium Base                         Cladding Metal Having the Formula                                             Nb--Ti.sub.40-48 --Al.sub.12-22 --Hf.sub.0.5-6                                Alloy           Nominal Alloy Additions                                       Designation     in Weight %                                                   ______________________________________                                        FS80            1 Zr                                                          C103            10 Hf, 1 Ti, 0.7 Zr                                           SCb291          10 Ta, 10 W                                                   B66             5 Mo, 5 V, 1 Zr                                               Cb752           10 W, 2.5 Zr                                                  C129Y           10 W, 10 Hf, 0.1 Y                                            FS85            28 Ta, 11 W, 0.8 Zr                                           SU16            11 W, 3 Mo, 2 Hf, 0.08 C                                      B99             22 W, 2 Hf, 0.07 C                                            AS30            20 W, 1 Zr                                                    ______________________________________                                    

Each of these commercially available alloys contains niobium as itsprincipal alloying ingredient and each of these alloys has a bodycentered cubic crystal structure. Each of the alloys also contains theconventional assortments and concentrations of impurity elementsinevitably present in commercially supplied alloys.

It will be understood that a reinforcing member can be formed from acombination of these alloys. For example, a reinforcing member can beformed as a composite of two or more strips, sheets, formed of twodifferent alloys, which may be combined into a composite structure to beincluded within a cladding of the niobium base cladding metal as setforth above. In this way it is possible to have a combination of metalswhich have a unique combination of properties, for example, at differenttemperatures, and in this way to provide reinforcing structures adaptedto serve unique functions within the clad outer envelope. A specificexample of such a structure is one in which a composite of B66 and FS85is clad by the cladding metal as set forth above. This combination wouldhave better specific strength (strength divided by density) than wouldclad FS85 alone at 2000 F., and would have better specific strength thanwould clad B66 alone at 2400° F.

The alloys of the above listing are alloys which are deemed to havesufficient high temperature strength and low temperature ductility toserve as the reinforcing element in composite structures having aniobium-titanium cladding as described above and having a composition asset forth in the following expression:

    Nb--Ti.sub.40-48 --Al.sub.12-22 --Hf.sub.0.5-6.

The form of the reinforcing elements of the strengthening alloy is aform in which there is preferably one small dimension. In other words,the strengthening element may be present as a rod or strip in which casethe reinforcement has one large dimension and two small dimensions, orit may be present as a plate or a sheet-like element or elements, inwhich case the reinforcing structure has one small dimension and twolarger dimensions.

A number of additional examples illustrate alternative methods ofpreparing the matrix composites of the cross-referenced applications.

EXAMPLE 3

A matrix-type composite structure was prepared by coextruding a bundleof round rods of matrix and reinforcement alloys.

The matrix (designated alloy 6) of the composite to be formedrepresented about 2/3 of the number of rods in the bundle andaccordingly 2/3 of the volume of the composite. This matrix metal had atitanium to niobium ratio of 0.5.

The matrix contained 27.5 atom percent of titanium, 5.5 atom percentaluminum, 6 atom percent chromium, 3.5 atom percent hafnium, and 2.5atom percent vanadium and the balance niobium according to theexpression:

    Nb--Ti.sub.27.5 --Al.sub.5.5 --Cr.sub.6 --Hf.sub.3.5 --V.sub.2.5.

The rods of the reinforcing component of the composite were of an AS-30alloy containing 20 weight percent (11.1 atom percent) of tungsten, 1weight percent (1.1 atom percent) of zirconium, (11.1 a/o W, 1.1 a/o Zr)and the balance niobium according to the atom percent expression:

    Nb--W.sub.11.1 --Zr.sub.1.1.

Approximately 70 rods of reinforcement and 140 rods of matrix havingdiameters of 60 mils each were employed in forming the matrix composite.The 210 rods were placed in a sleeve of matrix metal. The sleeve andcontents were enclosed in a can of molybdenum to form a billet forextrusion. The assembled billet and its contents were then processedthrough a 10 to 1 ratio extrusion. A section of the extruded product wascut out and this section was reprocessed again through a 10 to 1 ratioextrusion. A double extrusion of the rods was thus carried out. In doingso, the rods used as the starting materials were converted to fibers.

Following the double extrusion, the nominal size of each reinforcingfiber was about 150 μm . FIG. 8 is a micrograph of a portion of thecross-section of the structure. It is evident from the micrograph thatthe rods had lost their identity as round rods. Further, the veryirregular shape of the resulting strands formed from the rods within thecomposite had demonstrated that in a number of cases the elements whichstarted as rods were deformed and in some cases joined with otherelements to form the irregular pattern of matrix strands andreinforcement strands which is found in the micrograph of FIG. 8. Thisirregular cross-section resulted because no effort was made to restrainlateral movement of the rods during the extension, such as by shapingthe extended rods or by filling the interstices with smaller diameterrods.

Standard tensile bars were prepared from the composite and from thematrix material and tensile tests were performed. The results are setforth immediately below in Table V.

                                      TABLE V                                     __________________________________________________________________________    Tensile Results of Continuous Fiber Reinforced and Matrix Alloys                              Temp                                                                              0.2% YS                                                   Ex.                                                                              Sample                                                                             Alloy   (C.)                                                                              (ksi)                                                                              UTS (ksi)                                                                           EL.sub.ml (%)                                                                      EL.sub.f (%)                                                                       RA (%)                               __________________________________________________________________________            Composite                                                             3  91-12/A                                                                            AS-30/Alloy 6                                                                          70 121.0                                                                              121.0 0.2  0.2  1.5                                     91-12/B                                                                            AS-30/Alloy 6                                                                         760 78.1 89.3  4.8  20.6 27.0                                    91-12/C                                                                            AS-30/Alloy 6                                                                         980 43.7 44.3  3.8  48.5 50.0                                    91-12/D                                                                            AS-30/Alloy 6                                                                         1200                                                                              22.5 25.4  2.7  65.5 56.0                                         Matrix                                                                   91-32                                                                              Alloy 6  70 132.4                                                                              132.4 0.1  23.5 46.0                                    91-32                                                                              Alloy 6 760 83.1 92.1  1.7  48.3 64.0                                    91-32                                                                              Alloy 6 980 42.1 42.7  0.3  95.2 95.0                                    91-32                                                                              Alloy 6 1200                                                                              20.4 20.4  0.2  83.2 57.0                                 __________________________________________________________________________

The yield strength data of this table is plotted in FIG. 9.

It is apparent from a comparison of the composite and matrix data ofTable V that the matrix composite has lower strength than the matrixalone at lower temperatures but has higher strength than the matrixalone at higher temperatures. The ultimate strength of the matrixcomposite is about 20% higher than that of the matrix alone at the 1200°C. testing temperature.

Additional tests of the composite and of the matrix were carried out todetermine comparative resistance to rupture. Test results are presentedin Table VI immediately below.

                  TABLE VI                                                        ______________________________________                                        Rupture Results of Continuous Fiber Reinforced and                            Matrix Alloys                                                                                          Temper-                                                                       ature  Stress                                                                              Life                                    Ex.  Sample  Alloy       (C.)   (ksi) hours                                   ______________________________________                                                     Composite                                                        3    91-12   AS-30/Alloy 6                                                                              980   12.50 1282.36                                      91-12   AS-30/Alloy 6                                                                             1100   8.00  1928.20-                                                                      Test Stopped                                         Matrix                                                                91-32   Alloy 6      980   12.50 1.86                                         91-32   Alloy 6     1100   8.00  0.57                                    ______________________________________                                    

A comparison of the data for the composite and the matrix makes clearthat a highly remarkable improvement is found in the composite at bothtest temperatures. The improvement at the higher, 1100° C., testtemperature is of the order of thousands of percent. In fact, the testwas stopped because the beneficial effect of the reinforcement wasalready fully demonstrated.

The form of the reinforcement for the above examples is essentiallycontinuous in that the reinforcement and the matrix are essentiallycoextensive when examined from the viewpoint of the extended reinforcingstrands. Such composites are referred to herein as continuous compositesor as matrix composites having continuous reinforcing members.

The reinforcement of these matrix composite structures is distributed inthe sense that it is in the form of many elements having at least onesmall dimension. Such elements are referred to as strands ofreinforcement. Such strands may be in the form of ribbon or ribbonsegments or fibers or filaments or platelets or foil or threads or thelike, all of which have at least one small dimension and all of whichare referred to herein as strands.

As indicated above, the matrix composite structures are described in thecopending application Ser. No. 07/816,165. The structures of thecopending application are characterized generally by having largernumbers of smaller dimensioned, dispersed reinforcing elements containedwithin a matrix. Moreover, the smaller reinforcing elements of thecopending application are well distributed within the matrix.

By contrast, the structures of the present invention are clad compositestructures and the reinforcing element of these clad compositestructures is not subdivided and distributed within the structure. Thereinforcing elements are generally located centrally of the cladcomposite structure and are not subdivided to optimize their externalsurface area. In this respect the reinforcement of the clad compositestructures of this invention are not distributed in the same sense thatthe divided reinforcing structures of the copending applications aredistributed. The structures of this invention are distinct in that theyare clad structures in that there is a single core element, or aplurality of core elements, each of which has a relatively large bulkfor the amount of surface area containing the bulk. This contrasts withthe distributed reinforcing elements of the copending application inthat they can be multitudinous in number in the structure of thecopending application Ser. No. 816,165 and their dimensions and overallsurface areas are considerably smaller than they are in this subjectapplication. The findings based on the experiments reported aboverelating to matrix composites are deemed to be applicable to the cladcomposites of this invention.

One way in which the difference in the reinforcing structures of thematrix-type composite as contrasted with the clad-type composite of thepresent invention may be brought out is by reference to the reinforcingfactor R as discussed briefly below.

Generally the reinforcement of a clad composite must remain asreinforcement during the use of the composite article. By this is meantthat the dimensions of the reinforcement within the matrix must besufficiently large so that the reinforcing element does not diffuse intothe matrix and lose its identity as a separate niobium based alloy. Theextent of diffusion depends, of course, on the temperature of thecomposite during its intended use as well as on the duration of theexposure of the composite to a high temperature during such use. In thecase of a composite formed of a cladding having a melting point of about1900 degrees centigrade and a reinforcing phase having a melting pointof about 2475 degrees centigrade, an initial estimate, based onconventional calculations is that such a composite structure havingreinforcement elements of about 100μ in diameter or thickness would bestable against substantial interdiffusion for times in excess of 1000hours at 1200 degrees centigrade, and for times approaching 1000 hoursat 1400 degrees centigrade. Of course, the reinforcing core structuresof clad composites are generally much larger than the 100μ and thisdiffusion phenomenon does not present as large a problem for cladcomposites as it does for the matrix composites of copending applicationSer. No. 07/816,165, as referred to above.

Accordingly where the composite is to be exposed to very hightemperatures it is preferred to form the composite with reinforcingelements having larger cross sectional dimensions so that anyinterdiffusion which does take place does not fully homogenize thereinforcing elements into the matrix. The dimensions of a reinforcingelement which are needed for use at any particular combination of timeand temperature can be determined by a few scoping experiments and fromconventional diffusivity calculations since all of the parameters neededto make such tests, calculations and determination, based on the abovetext, are available to the intended user. Thus a reinforcing elementhaving cross sectional dimensions as small as 5 microns can be usedeffectively for extended periods of time at temperatures below about1000 degrees centigrade. However the same reinforcing element will behomogenized into the matrix if kept for the same time at temperaturesabove 1400 degrees centigrade.

Generally for matrix composites of the cross-referenced applicationslisted above, it is desirable to have the reinforcing elementsdistributed within the matrix so that there is a relatively largeinterfacial area between the matrix and the reinforcing elementscontained within the matrix. The extent of this interface dependsessentially on the size of the surface area of the containedreinforcement. A larger surface area requires a higher degree ofsubdivision of the reinforcement.

As a convenience in describing the degree of subdivision of thereinforcement within a matrix composite, a reinforcement ratio, R, isused as explained in copending application Ser. No. 07/816,165, filedJan. 2, 1992. As explained, the reinforcement ratio, R, is the ratio ofsurface area of the reinforcement in square centimeters to the volume ofthe reinforcement in cubic centimeters. The reinforcement ratio is thusexpressed as follows: ##EQU1##

As an illustration of the use of this ratio consider a solid cube ofreinforcement measuring one centimeter on an edge. This is one cubiccentimeter of reinforcement. Its ratio, R, is the 6 square centimetersof surface area divided by the volume in cubic centimeters, i.e., 1 cc.So the ratio, R, is equal to 6. For a cube of reinforcement measuring 2centimeters on an edge the surface area for each of the six surfaces ofthe cube is 4 square centimeters for a total of 24 square centimeters.The volume of a cube which measures two centimeters on an edge is eightcubic centimeters. So the ratio, R, for the two centimeter cube is 24/8or 3. For a cube measuring three centimeters on an edge the ratio, R, is54/27 or 2. From this data it is evident that as the bulk ofreinforcement within a surface keeps increasing (and the degree ofsubdivision keeps decreasing) the ratio, R, keeps decreasing. Pursuantto the present invention what is sought is a composite structure havinga lower degree of subdivision of the reinforcement rather than thehigher degree described in copending application Ser. No. 07/816,165.Generally, a reinforcement ratio of less than 50 is sought in cladcomposite structures of the present invention.

As a further illustration of the use of this ratio, consider a slab ofreinforcement which is embedded in matrix and which is more distributedrather than less distributed as in the above illustration. The slab canbe, for example, 40 cm long, 20 cm wide and 1 cm thick. The surface areaof such a slab is 1720 sq cm and the volume is 800 cubic cm. Thereinforcement ratio, R, for the slab is 1720/800 or 2.15. If thethickness of the slab is reduced in half then the ratio, R, becomes1660/400 or 4.15. If the thickness of the slab is reduced again, thistime to one millimeter (1 mm), the ratio, R, becomes 1612/80 or 20.15.Each of these structures when enveloped in a cladding as described aboveis within the scope of the present invention.

It should be understood that the reinforcement ratio, R, does notdescribe, and is not intended to describe the volume fraction, nor theactual amount, of reinforcement which is present within a composite.Rather the reinforcement ratio, R, is meant to define the degree of andthe state of subdivision of the reinforcement which is present, and thisdegree is expressed in terms of the ratio of the surface area of thereinforcement to the volume of the reinforcement. A further illustrationof the degree of subdivision of a body of reinforcement may be helpful.

As indicated above, a single body of one cubic centimeter ofreinforcement has a surface area of 6 sq. cm. and a volume of 1 cubiccentimeter (1 cc.). If the body is cut vertically parallel to itsvertical axis 99 times at 0.1 mm increments to form 100 slices each ofwhich is 0.1 mm in thickness, the surface area of the reinforcement isincreased by 198 sq. cm.(2 sq. cm. for each cut) but the volume of thereinforcement is not increased at all. In other words the degree ofsubdivision, and hence the surface area, of the body has been increasedbut the volume has not been increased. In this illustration thereinforcement ratio, R, is increased from 6 for the solid cube to 204for the sliced cube without any increase in the quantity ofreinforcement.

Pursuant to the present invention it is desirable to have thereinforcement in a bulk form so that the reinforcement ratio is lowerrather than higher. A reinforcement ratio, R, below 50 is desirable.

Also it is desirable to have the bulk reinforcement extend within thematrix to all those portions in which the improved properties aresought. For many clad composite structures the reinforcement should notextend to the outermost portions as these portions are exposed to theatmosphere. The cladding metal is the only metal of the clad compositewhich should be on the exterior of the composite and which should beexposed to the atmosphere at elevated temperatures. The outermostportions should preferably be the more protective cladding alloy:

    Nb--Ti.sub.40-48 --Al.sub.12-22 --Hf.sub.0.5-6.

Further, the reinforcement may be present in a volume fraction ofgreater than half of the composite. In this regard it is important thatthe cladding constitute the continuous external phase of the compositeand not be a discontinuous phase.

The claimed structures of the copending cross-referenced applicationslisted above are characterized by a high level of interfacial surfacebetween the matrix material and the reinforcing material embedded withinthe matrix. Depending on the temperature at which a composite is to beused, the presence of such large interfacial areas may not always beadvantageous. Where the use temperature is at the higher level, the factthat the two phases of the composite are mutually soluble can mean thatprolonged exposures at high temperatures will cause the eventualdisappearance of the composite and the composite will be replaced by anew, single phase composition of body centered cubic alloy.

The fact that both the matrix and the reinforcing alloy are bodycentered cubic is very important in the compositions of the listedcross-referenced applications as well as those of the presentapplication in that the formation of intermetallic compositions, havingother crystal forms and having inferior properties such as lowductility, is avoided.

In general, for matrix composites of the copending applications listedabove, where the reinforcement ratio R is high and the distribution ofthe reinforcing alloy is of a fine scale, then the lower is thetemperature and the shorter is the time of exposure before the matrixcomposite strengthening is reduced as the reinforcing elements aredissolved.

This latter relationship of size of reinforcing elements to usetemperature and time can be illustrated with reference to structuralconstraints actually found in jet engine parts. For many enginecomponents as used in the hot section of a jet engine, the wallthickness may be in a range of 0.04 inches or less. To maintain a 0.004to 0.005 inch layer of oxidation resistant metal at the surface, thecomposite core may be restricted to the order of 0.03 inches. In suchstructures the use of highly distributed reinforcing alloy presents aproblem of dissolution and eventual disappearance of the reinforcingelements because of the diffusion which occurs and because of the mutualsolubility of the matrix and reinforcing elements. For the highestservice times and temperatures accordingly it is difficult, because ofsuch restrictions, to achieve the very large interfacial surface areaswith high reinforcement ratios above 50 and above 100 as prescribed inthe pending cross-referenced patent applications.

Pursuant to the present invention, the structures which are provided arestructures which are useful for the highest temperatures and the longesttime of service. These structures may have a single layer ofreinforcement and a cladding layer enveloping the reinforcement formedof the preferred lower density cladding alloy having a higher resistanceto oxidation. The single layer of reinforcement may be present as acontinuous sheet or as a layer of multiple side by side strip-likephases or as a layer of pancake-like reinforcements or as a perforatedsheet or other structures that lend themselves to particularapplications within the engine structure. Such structures allow forreinforcement thicknesses which are much greater than for themultilayered configuration of foils, ribbons, fibers, etc. as describedand said forth in the pending matrix based applications listed in thecross-reference section above. Because of the greater reinforcement orcore thickness, these clad structures should survive thermal excursionsto higher temperatures for longer times without substantial loss of thereinforcement strengthening due to interdiffusion.

What is provided for this illustrative exemplary thin reinforced sheetstructure pursuant to the present invention is a thin clad structurehaving a higher density, stronger, reinforcing, niobium alloy embeddedin a single layered configuration of sheet strip or other form, within alower density, lower strength niobium based alloy cladding having acomposition according to the following expression:

    Nb--Ti.sub.40-48 --Al.sub.12-22 --Hf.sub.0.5-6.

What is claimed is:
 1. A composite article adapted for use attemperatures of 1000° C. or above comprising at least one ductilereinforcing element having a reinforcement ratio of 50 or less formed ofa niobium base alloy of body centered cubic crystal form, bonded to acladding of an alloy comprising in atom percent Nb_(balance) --Ti₄₀₋₄₈--Al₁₂₋₂₂ --Hf₀.5-6, said composite article being ductile and havinghigher tensile strength and rupture strength above 1000° C. than thecladding alloy.
 2. The article of claim 1 wherein the cladding alloycomprises, in atom percent,Nb--balance Ti--40 to 48 Al--12 to 20.Hf--1.5 to 5.5.
 3. The article of claim 1 where the cladding alloycomprises in atom percent:Nb=balance Ti=40 to 48 Al=12 to
 20. Hf=3.5 to5.5.
 4. An article according to claim 1 wherein the reinforcing elementcomprise at least 50 volume percent of the article and a thickness of atleast 100 microns.
 5. An article according to claim 1 wherein theniobium base alloy comprises, in atom percent:tungsten=11.1zirconium=1.1 niobium=balance.
 6. An article according to claim 1 inwhich the niobium base alloy comprises in weight percent:tungsten=9hafnium=30 niobium=balance.